The present invention generally relates to methods and apparatus for determining the heart rate of a subject, and particularly relates to methods and apparatuses for determining the beat-to-beat heart rate of a fetus.
Fetal monitoring (i.e., monitoring of the fetal condition during gestation and at birth) usually comprises monitoring uterine activity and the fetal beat-to-beat heart rate. The fetal heart rate, which provides an indication of whether the fetus is sufficiently supplied with oxygen, is preferably calculated from beat to beat.
To obtain a signal indicative of the fetal heart rate prior to rupture of the membranes, a noninvasive monitoring technique must be used. The most widely adopted measurement technique involves measuring the Doppler shift of an ultrasound signal reflected by the moving fetal heart.
In accordance with a known ultrasonic detection technique, an ultrasound transducer array is placed externally on the pregnant woman's abdomen and oriented such that the transmitted ultrasound waves impinge upon the fetal heart. The reflected ultrasound waves are received either by the same or by a different ultrasound transducer array. The Doppler shift of the reflected ultrasound wave is directly related to the speed of the moving parts of the heart, e.g., the heart valves and the heart walls.
To extract the Doppler component from the received ultrasound signal, typically the latter is demodulated. Further processing depends on the specific application. One technique for acquiring an acoustic indication of the heart beat uses autocorrelation. In accordance with the autocorrelation technique, the Doppler signal or the envelope of the Doppler signal is correlated with itself, thus providing significant peaks in time intervals which correspond to periodic components of the Doppler signal that are caused by the fetal heart beat. Such techniques are necessary because the received ultrasound signals contain noise originating from various physiological sources, such as the maternal aorta, movement of the fetus as a whole, and the like.
In prior art devices, a peak trigger device or an equivalent circuit detects the peaks in the autocorrelation function. The beat-to-beat heart rate, computed as the inverse of the time interval between two successive heart beats, is then available for further processing, display and/or recordation.
In clinical applications, the ultrasound transducer array is placed on the maternal abdomen such that the ultrasound waves will impinge upon the fetal heart. The transmitted ultrasound energy is typically in the form of bursts of high-frequency (e.g., 1 MHz) waves. In prior art devices, these bursts are very long in duration, which severely limits the spatial specificity of the resulting measurement. The final effect of this arrangement is to create a system that is more like a continuous wave ultrasound Doppler than it is like a pulsed wave Doppler device. The ultrasound waves reflected by the fetal heart and other fetal tissue are then received by ultrasound transducer array and fed to a demodulator, which produces a signal that is indicative of the Doppler shift of the received ultrasound signal relative to the transmitted signal. This Doppler shift is caused by the moving parts of the fetal heart, in particular the heart walls and the heart valves. The output of the demodulator is then fed to a bandpass filter that removes unwanted components in the Doppler signal. This filtered signal is then fed to an envelope detector, which generates the envelope of the peaks of the Doppler signal. Later the envelope of the Doppler signal is fed to an autocorrelation circuit that correlates the received signal with itself. The result of the autocorrelation function is that periodic components in the received signal are amplified, while non-periodic or stochastic signals (such as signals caused by movement of the fetus or the mother) are largely suppressed,
The output of the autocorrelation function is used to calculate the fetal heart rate. The significant peaks in the amplitude of the autocorrelated signal correspond to fetal heart beats and successive peaks are separated by time intervals that are approximately equal. The fetal heart rate is calculated as the inverse of the time interval between two successive peaks in the autocorrelated signal. Alternatively, the heart rate could be calculated by, e.g., computing it over a longer time interval and averaging the time intervals or averaging the heart rate.
One challenge in clinical applications is ensuring that the ultrasonic transducers are properly positioned relative to the fetal heart. Known instrumentation uses a relatively cumbersome array of single-element ultrasound transducers designed to cover the entire maternal abdomen. Unfortunately, as the result of fetal or maternal motion, the alignment of these transducers is such that the transducers fail to capture some echoes from the fetal heart and there will be a loss of heart rate information. When this occurs, the attending nurse must readjust the positioning of the transducer array. The resulting frustration experienced by hospital staff members because of this inconvenience is apparently to the degree that the instrument is often ignored and the mother and fetus do not receive the benefit of monitoring or the assessment of fetal distress. Additionally, in many instances the mother is instructed not to move, so that the fetal heart rate monitor does not lose the reflected ultrasound signal. This is contrary to good practice, since the mother's motion is important in causing the onset of heavy labor. If the mother were able to move around and still have a working fetal monitor, labor could be shortened in many instances.
There is a need for a method and means for continuously monitoring the fetal heart rate without any operator intervention. The method and apparatus should be designed so that the mother is allowed free motion while being monitored. It is also desirable that a more reliable heart rate measurement be provided as compared to current systems.